The issue: Redroot pigweed

Redroot pigweed (Amaranthus retroflexus), an upright summer annual, can grow to six feet tall. It germinates through the warm, summer months and sends down a deep taproot, making it drought tolerant. The taproot is pink to red in color.

The oval leaves taper to a blunt point and are dark green in color, usually with a red tint. The underside of the leaves on young plants often have a purple color. The flowers are not showy, and are somewhat bristly.

Redroot pigweed spreads only by seed. One plant can produce up to 35,000 seeds. It prefers rich soils and is often found on cropland, pastures, ditch banks, shop yards, vegetable gardens and flower beds.

Integrated pest management options:

· Prevention: Keep ditch banks free of seed-producing plants; clean all equipment and vehicles of weed chaff; feed weed-free hay; manage pastures for good competition; establish desirable plants in disturbance areas.

· Mechanical: Tillage and hand pulling before flowering; a weed burner on small plants is effective.

· Cultural: Establish and encourage desirable vegetation; mulch flower beds at least three inches deep.

· Biological: While the feed value of pigweed is good, drought-stressed pigweed can cause nitrate poisoning in cattle, sheep and goats.

· Chemical: Pre-emergent must be reapplied throughout the summer; several broadleaf weed killers have redroot pigweed on the label. High temperatures (>85 F) within 72 hours after application may cause some herbicides to volatilize and drift to non-target plants. Be sure the target weed and crop or landscape situations are listed on the product label. Always read and follow herbicide label directions.

Combine them:

Redroot pigweed seeds can remain dormant up to twenty years—a long-term program is essential. Combine different IPM options over a period of years to help improve the effectiveness of your efforts.

For more information, contact Ron Patterson, University of Idaho Extension horticulture/agriculture educator in Bonneville County.